A conventional wind turbine generally comprises a gearbox to increase the rotational speed of its generator relative to its rotor shaft and to increase the pole change frequency. A gearbox comprises many components that are subject to wear and must be maintained or replaced at intervals. Complex and expensive bearing arrangements are also required to bear the extreme loads and torque acting on the rotor shaft and the gearbox. A direct-drive wind turbine has several features over such a conventional wind turbine, whereby a major feature is that a direct-drive turbine does not require such a gearbox. A direct-drive wind turbine requires fewer parts, is less complex, and more reliable. For these reasons, demand for direct drive wind turbines is increasing.
A direct-drive generator has a relatively large diameter and many magnets of altering polarity arranged along a circumference of a field magnet arrangement—usually an outside rotor—to allow for a sufficiently high pole-change frequency. The physical dimensions and weight of such a large generator pose problems during its assembly. Handling of the heavy, unwieldy and vulnerable components is complex and time-consuming, and is also hazardous, so that strict safety measures must be adhered to. This adds considerably to the overall time and cost required for the assembly of a direct-drive generator.
For example, stator ring segments must be handled with care, but each ring segment can weigh hundreds of kilos, making it difficult to manoeuvre into position ready for mounting to a stator bedframe, which usually comprises a frontplate and a backplate mounted to a hollow stator shaft. A stator ring segment generally comprises several coil sections with windings wrapped in isolating material, and end-sections shaped to be connected to end-sections of neighbouring ring segments. Usually, a stator ring segment is transported to the partially assembled stator using a hoisting or crane, and is connected to this by cables or chains. The ring segment must be suspended in place until it is mounted to the stator bedframe. Extreme care must be taken when the ring segment is suspended during transport so that the vulnerable parts such as insulation or connectors are not damaged or bent, and so that the isolating material is not damaged. Furthermore, care must be taken to avoid damage or distortion of bedframe connecting device that are necessary for connecting the ring segment to the stator bedframe, otherwise an exact mounting is not possible. Damage to any of these elements results in expensive repair work and holds up production.